1. Technical Field
The present invention relates generally to refrigeration systems, and in particular to refrigerated containers or enclosures that employ mechanical refrigeration systems to maintain goods at a desired temperature during transport or storage. More particularly, the present invention relates to an improved system and method employing endothermic storage material in conjunction with forced air mechanical refrigeration to provide a more efficient refrigerated storage containment unit.
2. Description of the Related Art
Transportable cargo containers are well known in the shipping industry and typically comprise generally rectangular containers adapted to be placed onto wheeled chassis, railroad cars, ocean-going ships, etc., or are manufactured as an integral wheeled trailer or railroad car. Many goods and foodstuffs require refrigeration during transport to prevent spoilage or degradation, and it is well known to provide cargo containers with cooling and thermal insulation systems in order to maintain the cargo within a desired temperature range. Polyurethane foam is typically installed as solid sheets or as an injected foam within the containment walls of refrigerated cargo containers to form a thermal insulation barrier against outside ambient temperatures. The selection of a heat extraction system for conventional refrigerated cargo containers is made in accordance with temperature control and thermal efficiency characteristics that are best suited for a particular cargo type, and furthermore, in accordance with transport and delivery logistics.
For long distance transport and delivery in which a minimum number of intermediate loading and unloading stops are made in transit, cooling is often provided by blowing or injecting crushed ice or other endothermic substances, including toxic or otherwise hazardous chemicals such as liquid nitrogen or frozen carbon dioxide crystals, into the cargo container. These so-called xe2x80x9cclosed doorxe2x80x9d cooling systems are well-suited for relatively uninterrupted end-to-end long distance deliveries, and are highly efficient in terms of electricity and fuel consumption required to maintain the cargo below a given temperature. However, the sharp thermal gradients resulting from injecting the refrigerant directly into the cargo space in a solid, liquid or gaseous state, results in damage to sensitive cargos such as produce and refrigerated medications. Furthermore, the direct contact between the refrigerant and moisture-laden cargo results in excessive condensation of water vapor which freezes around the refrigerant, thus forming ice sheets that pose convenience and safety problems when accessing the cargo container, and also significantly reduce the thermal exchange efficiency between the refrigerant and the cargo chamber. The need to maintain a controlled, low humidity atmosphere within the cargo chamber makes injected refrigerant techniques unsuitable for so-called xe2x80x9copen-doorxe2x80x9d transport in which refrigerated or frozen goods are delivered from a centralized source such as a warehouse to several dispersed retail destinations.
Many of the problems associated with the foregoing transport refrigeration techniques are addressed by mechanical refrigeration systems, which have consequently come into widespread use. Mechanical refrigeration systems used for transport employ a conventional evaporator/condenser thermal exchange mechanism that operates in accordance with many of the same refrigeration principles used in household refrigerators and freezers. The mechanical refrigeration system is typically attached onto one end of a refrigerated transport container, commonly referred to in the art as a xe2x80x9creefer,xe2x80x9d and includes a blower to produce a forced-air cooling effect within the container, inducing a convective mode of heat transfer required for quickly restoring and maintaining the prescribed cargo temperature in an open door transport context.
Compared with other transport refrigeration techniques, mechanical refrigeration results in minimal ice build-up in the cargo chamber and furthermore provides a more stable temperature control mechanism than that provided by injected refrigerant systems which are better suited for very low temperature storage. However, the compressors utilized in conventional mechanical refrigeration systems consume considerable electrical power that must be generated on-board for transportable cargo applications. Furthermore, the convective, forced-air thermal exchange mechanism, in which cold air is blown over the refrigerated cargo, inevitably results in an uneven temperature distribution and abrupt thermal gradients within the refrigerated container and-its cargo. In marine transport environments, utilization of mechanically refrigerated containers presents logistical problems relating to restrictions as to where the containers can be placed onboard the ship. Specifically, the air exhaust requirements of refrigerator condenser fans restricts the loading and placement of mechanically refrigerated containers to above-deck locations where the warm condenser exhaust is freely released into the open atmosphere without negatively impacting other nearby refrigerated containers. This locatability restriction naturally results in a competitive scarcity of available above-deck storage space, significantly increasing shipping costs for reefers. In addition to the foregoing problems relating to energy consumption, uneven thermal gradients, and shipping costs, the continuous cycling of mechanical refrigeration systems results in significant maintenance costs incurred in repairing and replacing worn or depleted refrigeration system components and related power generating components.
It can therefore be appreciated that a need exists for an improved cargo refrigeration system and method of implementing the same that address the foregoing problems associated with the conventional injected chemical refrigerant and mechanical forced-air cooling systems. The present invention addresses such a need.
A bimodal refrigeration system is disclosed herein. The present invention includes a refrigerated container having an enclosed cargo chamber for storing refrigerated goods and a mechanical refrigeration unit that produces a chilled forced-air circulation within the cargo chamber. The cargo chamber is designed to guide the forced-air in a specified discharge and return path. In accordance with the present invention, a row of thermal storage ducts for retaining endothermic storage material are disposed within the cargo chamber. The thermal storage ducts are longitudinally disposed in parallel with the air flow direction of the specified forced-air discharge and return path to enable maximum convective thermal exchange between the forced-air current and the endothermic storage material without obstructing the specified forced-air discharge and return path.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.